The present invention relates to a process for the preparation of pyridines and/or hydrogenated pyridines that have been substituted with a hydrocarbon group at one or more of the carbon atoms. Such compounds are useful, for instance, in the preparation of pharmaceutical products and insecticides.
It is known that these compounds can be prepared by a .gamma.-cyanoketone, in a gaseous phase and in the presence of hydrogen, over a catalyst containing a catalytically active metal or compound of a metal selected from the group consisting of copper, silver, gold, iron, nickel, cobalt, ruthenium, rhodium, palladium, osmium, iridium and platinum. A process of this type has been described in British Pat. Nos. 1,304,155 and 1,425,698. The gaseous reaction mixture obtained in this known process is predominantly hydrogen, usually containing from about 5 to 20% of other gases, and can be reused after being separated from the reaction mixture in the reaction of a further quantity of .gamma.-cyanoketone.
It has been found, however, that when this hydrogen-containing gas is recycled, the selectivity and activity of the catalyst used in the reaction drops much more rapidly than it would without the recirculation of such hydrogen-containing gas.
It is therefore an objective of the present invention to provide an improved method for the preparation of substituted pyridines and/or hydrogenated pyridines wherein the hydrogen-containing gas separated from the resulting reaction mixture can be recycled without adverse effect on the selectivity and activity of the catalyst.